One of the many processes employed during the manufacture of paper and paper board products involves the color coating of the paper products. The color coating procedure is used to enhance both the aesthetics and printability of the paper products. This procedure involves applying a particular coating color formulation onto a paper or paper board web as it moves at varying speeds through a coating apparatus. Examples of material found in coating color formulations typically used in the paper industry include pigments, such as Kaolin clay, titanium dioxide, calcium carbonate, silicates, adhesive binders, such as starches, styrene butadiene latex, polyvinyl acetates, and additives, for either improving or modifying specific physical properties and characteristics of the coating color formulation.
In general, a lubricating additive in coating color formulations acts as a plasticizer, reducing the degree of interactions between dispersion particles and the continuous phase of a color formulation. This allows for a more effective separation of water and dispersion particles, thus enhancing exudation of the clay, lubricant and binder particles to the paper surface during coating. In terms of rheology, the color dispersion with the lowest elastic modulus (G') will contain the more effective lubricant. Thus, a preferred lubricant, when added to a paper coating color formulation, should decrease the elastic modulus G' and lower its viscosity to an optimum coating level. On a relative basis, the lubricant that exhibits the greatest loss tangent (G"/g') is ideal. However, the G' should decrease to a greater degree than G", allowing the material to behave more like a fluid than a solid. The rheology of the coating composition affects its runnability, blade pressure, leveling, streaking, shelf life stability, and dusting. Gloss, while affected by rheology, also depends on other parameters such as glass transition temperature, crystallinity, particle size and UV absorption characteristics. Therefore, it is an object of the present invention to provide a lubricating additive which, when added to a standard paper coating color formulation, improves the coating composition's rheology.
One way of determining the effectiveness of a paper coating color formulation is by looking at its "runnability". Runnability is defined in terms of how easy it is to apply a color coating formulation onto paper or paper board during the coating process. Ease of coating is defined by determining either: (1) the lowest blade pressure required to obtain a desired coat weight at the highest coating speed, or (2) the lowest blade pressure as a function of coat weight at constant coating speed for a series of coating color formulations. Consequently, a coating color formulation capable of providing a desired coat weight, at a low blade pressure, and at a high coating speed, on a consistent basis, is deemed to have superior runnability.
A typical paper coating device used in the industry is a short dwell coater. This device applies a coating color formulation onto a paper or paper board web passing through it under pressure, using an instrument known as a doctor blade to control the thickness of the coating being applied onto the web. The paper or paper board web travels through the short dwell coater at a speed of approximately five thousand feet per minute, thus causing a tremendous amount of pressure to be applied onto the blades, since the amount of pressure is a function of velocity. As a result, the coating blades must be changed every few hours. Improving runnability will enable a reduction in blade pressure, thus resulting in significant cost savings relative to equipment wear and downtime. Therefore, it is an object of the present invention to provide a coating composition capable of reducing the blade pressure needed to be applied when coating paper.
Runnability is also determined by a physical inspection of the coated paper and paper board web. A number of undesirable chemical phenomena and physical forces associated with the color coating process detrimentally affect the integrity and uniformity of both the coating color formulation and coating weight being applied. Such problems are commonly encountered in the paper manufacturing industry and result in the costly waste of materials, equipment, manpower, and product. A poorly coated sheet will exhibit uneven coat weight along the cross web direction for a significant distance along the run. This phenomenon is commonly referred to in the industry as a "streak". This is caused when the viscosity of the coating color formulation is too low or the elastic modulus is too high. Conversely, when the viscosity of the coating color formulation is too high, blade pressure must be increased in order to obtain the proper coating weight causing the blade to wear prematurely. Moreover, the above mentioned problems are magnified as the speed of the coating apparatus is increased. Problems relating to viscosity maintenance, calcification, streaking, and whiskering, all of which are considered factors used in determining the runnability or effectiveness of the paper coating color formulation, as it is applied onto paper, are enhanced when the speed of the coating process is increased. These problems ultimately result in a poor quality coated sheet. Hence, obtaining a coating color formulation having optimum viscosity is critical to achieving improved runnability.
Another problem associated with the paper coating process is "dusting", which is defined as the degree of contaminant debris left behind on the supercalender rolls after glossing a given amount of paper. This debris consists of the components of the dry coating color formulation. If the viscosity of the coating color is too low, coating color dewatering will occur, causing binder migration into the base sheet. This will increase the possibility of dusting.
A compound typically employed by the industry in an attempt to overcome these problems during paper coating runs is calcium stearate. This compound, although somewhat effective, fails to significantly inhibit the occurrence of the above-mentioned chemical and physical phenomena. Moreover, its effectiveness is diminished as the speed of the coating process is increased. Therefore, it is an object of the present invention to provide a paper coating composition capable of improving runnability while decreasing the incidence of dusting.
The effectiveness of a lubricant additive when combined with a coating color formulation is also determined by measuring its dry state paper properties. While many dry state paper properties are measured, a few of which include brightness, opacity, and wet and dry pick, the most important for lubrication are gloss and the above-mentioned dusting.
Gloss is a measure of the specular reflected light from a coated sheet of paper at a 75 degree incident angle. A typical gloss value for a coated sheet of lightweight coated paper (LWC) is 50 at approximately 6#/ream coat weight. An improvement of 1 or 2 gloss units is considered significant. Gloss is dependent on coat weight. Therefore, it is an object of the present invention to provide a lubricant additive capable of reducing dusting while increasing gloss during a paper or paper board coating process.